1. Field of the Invention
This is a method for manufacturing a cable for use in seismic exploration. The cable is designed to include along its length, spaced-apart clusters of multi-axis geophones. Each cluster includes at least two multi-axis geophones that are secured to opposite sides of the cable. Seismic signals resulting solely from cable rotation about its in-line axis, are expected to cancel.
2. Discussion of Relevant Art
As is well known to geophysicists, in the conduct of a seismic survey, a sound source, at or near the surface of the earth, is caused periodically to inject an acoustic wavefield into the earth at each of a plurality of regularly-spaced survey stations. The wavefield radiates in all directions to insonify the subsurface earth formations whence it is reflected back to be received by seismic sensors (receivers) located at designated stations at or near the surface of the earth. The seismic sensors convert the mechanical earth motions, due to the reflected wavefields, to electrical signals. The resulting electrical signals are transmitted over a signal-transmission link of any desired type, to instrumentation, usually digital, where the seismic data signals are archivally stored for later processing. The travel-time lapse between the emission of a wavefield by a source and the reception of the resulting sequence of reflected wavefields by a receiver, is a measure of the depths of the respective earth formations from which the wavefield was reflected.
The seismic survey stations of a 3-D survey are preferably distributed over a regular grid (preferably rectangular) of intersecting lines of survey over an area of interest. The inter-station spacing is on the order of 25 meters. Every station is preferably occupied by a seismic receiver; the acoustic-source spacing is usually an integral multiple of the receiver-station spacing.
The seismic receivers transmit data to the preferred instrumentation by data communication channels that are contained within a seismic cable which may be tens of kilometers long. Because of their great length, a seismic cable is usually made in sections, each about 100 meters long. Each end of a section is provided with a connector for providing mechanical and data-communication linkage between the desired plurality of sections. The data transmission channels in the cable may be electrical, over wire lines, or optical, via optical fibers, analog or digital, multiple-channel or multiplexed single-channel.
Acoustic wavefields propagate through the earth in various modes. Of interest are compressional waves and shear waves. Compressional waves are polarized in-line outwardly away from the source. Shear waves are polarized horizontally both in-line and cross-line directions. Therefore, when all three wavefields are of interest in a survey, multi-component receivers are used. For such surveys, each receiver package includes a three-component (tri-axial) receiver: a vertically-polarized receiver responsive to compressional wavefields and two shear-wave receivers that are polarized horizontally in the in-line and the cross-line directions respectively.
Three-component receivers are customarily provided in a single cylindrical package which may be mechanically coupled to the seismic cable at desired intervals. The receiver signal output terminals are linked to the data transmission channels. With particular reference to ocean bottom cables (OBC), the respective multi-component receiver packages are usually integrated directly into the cable.
Seismic cables are cylindrical. In shallow water, on the bottom, sea currents cause the elongated cable to roll slightly around its in-line or longitudinal axis. Any wavefield that has a polarization component in the cross-line direction can cause the cable to oscillate slightly around its in-line axis. The in-line receiver is not affected but the cable-rolling motion introduces severe noise to the signals originating from the cross-line and the vertically-polarized receivers.
Although several mathematical data-processing techniques are known for reducing signal distortion due to poor ground coupling, those techniques are not completely satisfactory. Since there is no practical way to physically prevent ocean-bottom cable movement, there is a need for a method for manufacturing an OBC cable that will inherently provide noise cancellation ab initio in the presence of undesired cable motions.